The invention relates to a low conductivity thermal barrier coating, in particular to a low conductivity thermal barrier coating containing zirconia and/or hafnia, and a combination of rare earth oxides and/or certain other oxides.
If Thermal barrier coatings are thin ceramic layers that are used to insulate air-cooled metallic components from high temperature gases in gas turbine or other heat engines. Such coatings are useful in protecting and extending the service life of metallic components exposed to high temperatures, such as jet engine turbine blades. Thermal barrier coatings composed of yttria-stabilized zirconia are known, wherein the yttria typically makes up seven to nine weight percent (or four to five molar percent) of the total composition. These coatings are typically applied using plasma spraying or physical vapor deposition process in which melted ceramic particles or vaporized ceramic clouds are deposited onto the surface of the component that is to be protected. Thermal barrier coatings are somewhat porous with overall porosities generally in the range of 5 to 20%. This porosity serves to reduce the coating""s thermal conductivity below the intrinsic conductivity of the dense ceramic, as well as to improve the coating""s strain tolerance. However, the coating conductivity will increase as the porosity decreases in high temperature service due to ceramic sintering.
In a jet engine, higher operating temperatures lead to greater efficiency. However, higher temperatures also cause more problems such as higher stresses, increased materials phase instability and thermal oxidation, leading to premature failure of the component. A ceramic coating with lower thermal conductivity and improved high temperature stability would allow higher operating temperatures while preserving operating life of the coated component. Accordingly there is a need for thermal barrier coatings with a lower conductivity and better sintering resistance than prior art coatings. Such a coating ideally would retain low conductivity after many hours of high temperature service. A laser test, recently developed by the current inventors has allowed simultaneous testing of durability, conductivity, and conductivity increase due to sintering under turbine-level high heat flux conditions. Thus the thermal barrier coating advances described in this invention have had the benefit of this new test approach.
A thermal barrier coating composition is provided. The composition is about 46-97 molar percent base oxide, about 2-25 molar percent primary stabilizer, about 0.5-12.5 molar percent group A dopant, and about 0.5-12.5 molar percent group B dopant. The base oxide is selected from the group consisting of ZrO2, HfO2, and combinations thereof. The primary stabilizer dopant is selected from the group consisting of Y2O3, Dy2O3, and Er2O3 and combinations thereof. The group A dopant is selected from the group consisting of alkaline earth oxides, transition metal oxides, rare earth oxides and combinations thereof. The group B dopant is selected from the group consisting of Nd2O3, Sm2O3, Gd2O3, Eu2O3 and combinations thereof.